The preparation of microemulsions is one attempt that has been made to improve the absorption of physiologically active substances through the skin or mucous membrane.
Microemulsions have been proposed that use alcohols such as octanol and butanol. However, they are not particularly suitable for oral administration since the alcohols they use have malodors. If microemulsions are prepared using large amounts of ionic surfactants, they are irritant to the mucous membrane and skin.
Peptides such as insulin and calcitonin have low absorbability through the mucous membrane. With a view to dealing with this problem, the use of absorption enhancers such as bile salts has been attempted but they have been found to damage or destroy epithelial cells of the mucous membrane.